Subject: bug#18778: noticeable slowdown for buffers with long lines, word-wrap, and brackets

From: Eli Zaretskii <[email-masked]>
Date: Mon, 20 Oct 2014 22:33:12 +0300

> From: Ivan Shmakov <[email-masked]>
> Date: Mon, 20 Oct 2014 19:17:28 +0000
> 
> 	• create a buffer with long lines (see below for the example
> 	  I’ve used), and (setq word-wrap t line-move-visual nil) there;
> 
> 	• now, enclose every line in [, ] brackets (as in: M-x
> 	  replace-regexp RET .* RET [\&] RET; parentheses or curly
> 	  braces also exhibit the issue.)
> 
> 	For the resulting buffer, operations like (next-line) or even
> 	(recenter) now result in a noticeable delay.
> 
> 	The issue doesn’t appear when word-wrap is not used, or when
> 	there’s no brackets in the buffer.  Neither the issue appears in
> 	Emacs built 2014-10-09 from a then-recent Git clone.
> 
> 	I’ve used the output of the following Shell command as a test.
> 
> $ head -n 8192 < /usr/share/dict/american-english | fmt -w 1024 

Is it an important use case?  If so, what is the real-life situation
here?